A power supply system, a distributed power storage system, an electric vehicle and the like using a power storage device such as a battery are equipped with a battery control circuit managing a state of the power storage device. A state of charge (SOC) is a representative example of an index indicating the state of the power storage device managed by the battery control circuit.
As one method of estimating the SOC, a current value flowing into and out of the battery is measured and integrated. This method integrates a measurement error included in the measured current value as well, and thus has a problem that an SOC error increases as time elapses.
In order to solve the aforementioned problem, PTL 1 discloses the following invention.
A charge or discharge current through a secondary battery is integrated to obtain a first integrated value, which is divided by a battery capacity and then added to an initial SOC value to continuously calculate a first SOC value. A terminal voltage across the secondary battery obtained at a timing of switching an operation between charging and discharging is corrected to approach an open circuit voltage to then obtain the SOC at that point as a second SOC value, where the initial SOC value is updated by the second SOC value every time the second SOC value is obtained to resume the integration computation of the first integrated value.
According to the invention described in PTL 1, the SOC error increasing as the measured current value is integrated can be updated at the timing of switching charging/discharging, whereby the accuracy of the SOC is effectively maintained.